The present invention relates to a solid-state image sensing device, and in particular, relates to a solid-state image sensing device that transfers an image digital signal converted by an analog-digital converter from the analog-digital converter to a signal holding circuit.
In the past, film cameras have been in widespread use. However, with the development of digital processing technology in recent years, digital cameras have been replacing the film cameras. The digital cameras have been significantly improving image quality thereof, and the latest models of digital cameras have better image quality than the film cameras. A solid-state image sensor is a key device to improve the image quality of the digital camera.
The solid-state image sensor used in the digital camera is divided broadly into CCD (Charge Coupled Device) and CMOS (Complementary Metal Oxide Semiconductor) image sensors. Particularly, in terms of higher functionality in cameras, attention has been given to the CMOS image sensor which can easily mount peripheral circuits.
Further, the CMOS image sensor includes an analog image sensor which outputs image analog signals generated by photoelectric conversion of light received by imaging elements in a pixel array and a digital image sensor which outputs image digital signals into which image analog signals generated by photoelectric conversion are converted. Particularly, in terms of data processing speed, attention has been given to the digital CMOS image sensor which easily enables enhanced speed.
Specifically, due to the higher data processing speed, the digital CMOS image sensor enables not only moving image shooting but also various applications in combination with image processing. For example, the digital CMOS image sensor enables a camera to determine the moment when a tennis racket hits a ball and automatically trigger the shutter, or to determine the moment when a child crosses the finish line at an athletic meet and automatically trigger the shutter to shoot the face of the child. To perform such processing, in particular it is necessary to convert captured image analog signals into image digital signals at high speed and transfer the converted image digital signals to an image processing circuit at high speed.
However, in the CMOS image sensor, it is necessary to process massive data to convert the captured image analog signals into the image digital signals. For example, in the case where the CMOS image sensor having 10 million imaging elements (10 million pixels) shoots a moving image with a frame rate of 30 fps, if one analog-digital converter (ADC) is used for data processing, it is necessary to convert an image analog signal of one imaging element (one pixel) into an image digital signal of gradation and transfer the image digital signal to a register within 3 ns, which is difficult to achieve.
Accordingly, in an image sensor disclosed in Japanese Unexamined Patent Publication No. 2000-152082 (Patent Document 1), analog-digital converters are disposed in respective columns in a pixel array. For example, the CMOS image sensor having 10 million pixels is configured with 3900 pixels (3900 columns) in the horizontal direction and 2600 pixels (2600 rows) in the vertical direction, and analog-digital converters are disposed in the respective columns, thereby converting an image analog signal into an image digital signal and transferring the image digital signal to a register within 12.8 μm at maximum, which is feasible. Further, in the image sensor disclosed in Patent Document 1, the pixel array is divided into two groups, in each of which image analog signals are converted into image digital signals which are transferred to registers, thereby enabling faster data processing.